1. Field of the Invention
The present invention relates generally to multi-channel audio power output stages, and more specifically, to a circuit and method for sharing an AC blocking capacitor between two or more stages while reducing the size requirements of the blocking capacitor.
2. Background of the Invention
Audio power output stages are present in any audio rendering system that ultimately provides a signal to an audio transducer. When dual power supplies are not present or practical (i.e., when the power supply is not split), then the power stage output typically must be AC coupled through a DC blocking capacitor, so that DC currents are not generated through the transducers. Since there is no midpoint referenced return path in a single-supply amplifier system, the DC potential between the quiescent power stage output terminal voltage and either of the power supply rails is typically half of the power supply voltage.
In computer systems such as present-day personal computing devices, there are typically only uni-polar power supply outputs available, e.g., a high current 3V-5V supply and generally a lower-current 12V supply. There is a demand for audio power output stages in such systems so that separate amplifiers are not required for rendering audio produced by the computer system.
Further, in other low-cost consumer audio systems, requiring a split power supply increases the cost of the transformer and the cost of the filter bank for the power supply, or alternatively requires a bridge amplifier.
Inclusion of large DC blocking capacitors is not only costly, but consumes a significant amount of packaging volume, as in high power output stages having low-impedance transducers such as speakers, the DC blocking capacitors are typically very large, for example, aluminum electrolytic capacitors having a capacitance on the order of 2000 microfarads will yield a 3 dB corner frequency of around 20 Hz with a 4 ohm transducer. Since bass attenuation is unacceptable in most modern music applications, as well as for sound effects in video presentations, the size of the output DC blocking capacitors cannot be compromised.
Therefore, it would be desirable to provide a method and system for sharing a DC blocking capacitor among multiple audio power output stages. It would further be desirable to minimize the size of the DC blocking capacitor required in power amplifiers operated from a single power supply output.